Mostly, I tell the truth, but I can sometimes lie,
If you're skinny, fat, bald, or tall, I can tell you why.
Reassurance is what you seek from me, though I'm not the best,
Really, I can only look skin deep, I can't figure out the rest.
Odd, I stand before you, trapped, never to escape,
Returning your surprise, my mouth stands agape.
What am I?